Recently, the world's energy source is gradually depleted. Moreover, with the increasing environmental consciousness and the popularity of environmental protection, the demand on renewable energy is growing. Consequently, the conventional gasoline vehicles are gradually replaced by electric vehicles, and the electric vehicle has become popular transport means in recent years.
Nowadays, an electric vehicle such as a large electric vehicle is equipped with a power driving system and a motor. The power driving system is used for driving operations of the motor in order to drive the electric vehicle. Conventionally, the power driving system receives a constant DC bus voltage from a battery box, and a power transistor of the power driving system is operated in a pulse width modulation mode to generate a sine wave to drive the motor. The power transistor operated in the pulse width modulation mode can largely reduce the power loss and heat generation. However, if the rotating speed of the motor is very low, the duty cycle of the power transistor has to be reduced to a very low value. Under this circumstance, the sine wave outputted from the motor drive is suffered from distortion. The distorted sine wave will result in torque ripple. The torque ripple will adversely affect the motor power and the comfort of riding the electric vehicle.
Generally, as the DC bus voltage is reduced, the torque ripple resulted from the low motor speed can be overcome. However, the maximum motor speed is also decreased. A method of avoiding the generation of the torque ripple at the low motor speed uses a low voltage source, and a boost circuit is connected between the voltage source and the DC bus. In case that the motor speed is low, the voltage source directly provides electric energy to the DC bus. Whereas, in case that the motor speed reaches a specified speed, the boost circuit is enabled to increase the DC bus voltage. However, as the motor power is gradually increased, the cost of the boost circuit is increased and thus the fabricating cost of the power driving system is high. Moreover, since the power transistor of the boost circuit may generate a slight voltage drop, the efficiency is impaired.
Another method for reducing the torque ripple uses a voltage-switchable voltage source. Generally, the electric vehicle such as the large electric vehicle is usually equipped with plural batteries. In addition, the plural batteries are serially connected with each other through relays in order to achieve the voltage-switchable function. However, while the relays are switched, the generated electric arc may damage the contacts of the relays. Moreover, while the relays are switched, the generated surge current may adversely affect the use lives of the batteries.
Therefore, there is a need of providing an improved power driving system in order to overcome the above drawbacks.